H01C7/00—Non-adjustable resistors formed as one or more layers or coatings; Non-adjustable resistors made from powdered conducting material or powdered semi-conducting material with or without insulating material

ICAL RESISTORS AND METHODS OF PRODUCING THE SAME R. H. RESIN BONDED ELECTR United States The present invention relates to electrical resistors and to a method for producing these resistors. More particularly, the present invention is concerned with new and improved electrical resistors of the type which may be used in printed circuits.

This application is a continuation in part of US. patent application S.N. 402,702, filed January 7, 1954, now abandoned.

Resistor elements which comprise a dispersion of finely divided conductive particles in a solid dielectric such as, for example, a polymerized resin, are well known in the prior art. Generally, these resistors comprise a dispersion of finely divided carbon or metal particles in a solid resinous material which may additionally contain various organic or inorganic filler materials. The resins that are used in the production of such resistors may be of either a thermosetting or a thermoplastic nature depending, of course, upon the specific requirements of a particular installation. Natural resins such as shellac, as well as a variety of synthetic resinous materials such as phenolic condensation products, alkyd resins, vinyl resins etc. have been found suitable in the manufacture of such resistors as described above.

Aside from their use in the production of molded or cast resistors, these conductor filled resinous materials may be applied in thin layers onto a non-conducting base, i.e., a ceramic, glass, paper, fiber, or resin base. A variety of methods are currently employed in order to apply these thin coatings, such as for example stenciling, printing, painting or spraying directly upon the non-conducting base. In modern radio circuits, variable resistances of the latter type are used as tune and volume controls and are included in incident portions of the circuits. In addition, fixed resistances of this type are used in filter networks, audio frequency coupling stages, and in general whereever resistances occupying a minimum of space are desired.

Heretofore, the value of the resistance of any particular resistor material comprising conductive particles dispersed throughout a solid resinous dielectric could only be controlled or regulated by altering the proportion of the weight of dielectric used to the weight of conducted particles employed. Thus, a resistor having a low ratio of weight of resin, or resin plus filler, to the weight of conductive particles, had a low resistance. correspondingly, resistors having high values of resistance were obtained by increasing the proportion of dielectric material employed. Unfortunately, however, an increase in the dielectric-to-resin weight ratio was always found to result in the impairment of certain electrical properties of the resistor element. Notably, the value of the voltage coefficient as well as the noise factor of a resistor was seen in increase with an increase in the dielectric-to-conductor ratio. Accordingly, it was often extremely difficult if not impossible to obtain satisfactory conductor-filled resin resistors having high values of resistance without using different, and often more expensive materials than those employed in the manufacture of resistors of this nature having low resistance values.

Table I, below, is believed to be illustrative in this regard. The data presented therein pertains to tests con ducted on resistors comprising thin strips of resistance material printed on an insulating base. The dimensions of each strip were the same, the length being 0.187 inch atent Ofidce 3,056,750 Patented Oct. 2, 1962 and the Width being 0.062 inch. The thickness of the strips was determined by the process of printing, which in all cases was the same. The conductive material employed Was Sterling 99R Furnace black carbon supplied 5 by Godfrey L. Cabot, Inc. of Boston, Massachusetts.

The relatively good combination of electrical characteristics possessed by this carbon makes it a desirable material in the manufacture of printed resistors. The resin employed was Durez 7522, a liquid, one step thermosetting phenol-formaldehyde resin, having 90 percent solids at 135 C. and a viscosity of LOGO-3,000 centipoises at C. No inert, non-resin fillers were employed.

It becomes obvious from the above data that an attempt to increase the resistance of a resistor by increasing the 25 proportion of resin in the resin-conductor mix, leads to a deterioration of certain electrical properties. As will hereinafter appear, however, use of the present invention permits the production of resistors having resistivities of 1 megohm and above without the necessity of employing a high resin to conductor ratio.

A further problem inherent in the use of the conductorfilled resinous resistors of the prior art was their inability to maintain constant values of resistance after prolonged use at elevated temperatures. Thus, this type of resistor often proved to be entirely inadequate in various electronic circuit applications where the resistance value of resistance used had to be maintained at a fairly constant level.

Accordingly, it is the primary object of the present invention to provide a new and improved resistance material which possesses a high degree of stability and maintains a constant value of resistance after prolonged use at elevated temperatures.

A further object of the present invention is to provide a new and improved resistance material comprising conductive particles dispersed in a solid dielectric, the resistance of which may be varied throughout a wide range at a constant dielectric to conductor ratio.

Another object of the present invention is to provide a new and improved method for producing a resistor material which will retain a constant value of resistance after prolonged use at elevated temperatures.

While the novel and distinctive features of the present invention are particularly pointed out in the appended claims, a more expository treatment of the invention, in principle and detail, together with additional objects and advantages thereof is supported by the following description and accompanying drawings in which:

FIGURE 1 is a plot of the values of resistivities (ex- 60 pressed in megohms) of various resistors made in accordance with the present invention against the values of the corresponding resin to conductor ratio.

FIGURE 2 is a plot of the value of the ratio of megohms of a resistor to the weight of the resistor ma- 65 terial contained therein against the number of passes the material was subjected to on a Kent mill.

In accordance with the present invention, a resistor is provided which comprises discrete units dispersed in one or more solid resinous dielectrical materials. Each one 70 of these discrete units in turn comprises an aggregate of conductive particles which have been precoated, at least in part, with one or more dielectric materials which sub sequently may be polymerized, or partially polymerized so as to bind the individual conductive particles together to form the aggregate, or unit. By virtue of the polymerized, or partially polymerized resinous coating, the conductive particles are held in such a relation as to establish an electrical pathway of a certain definite resistivity through the aggregate. By means of subsequent manufacturing techniques, the discrete units are suspended and held in a polymerized resin dielectric, which resin may be the same or a different resin or resins from those used to precoa't the conductive particles, and which may contain one or more filler materials which also are dielectric and are thus maintained in such a relation as to provide an electrical pathway of a certain definite resistivity through the completed resistor.

Briefly stated, therefore, the new and improved resistor material of the present invention comprises a plu- 'rality of discrete units which are dispersed in a polymerized resin, each discrete unit comprising an aggregate of conductive particles of either carbon or metal, or both, which particles are, in turn, bonded together by a solid polymerized resinous material to form the discrete unit.

The discrete units above referred to are produced by adding the conductive particles to a solvent solution of one or more polymerizable resinous materials. The conductive particles are dispersed by means of milling, with orwithout the addition of dispersing agents, into a suspension of particles in the resinous solution. If desired, various fiocculating agents may also be added. The volatile solvent portion of the resulting mixture is then removed under conditions of heat or vacuum, or both, and the resulting residue, comprising the resin and the conductive particles, may be pulverized at this point or subsequent to the polymerization of the resin, or both. The residue is next subjected to heat in order to effect the polymerization of the resin. The resulting product comprises conductive particles which are coated with a thin film of polymerized resin. Subsequent pulverization reduces the product to a powder of the desired particle size.

The resulting powder is thus composed of discrete units, each discrete unit comprising an aggregate of conductive particles coated, and thus held in a fixed interrelationship, by the polymerized resin. Inasmuch as the resin coating has been polymerized in the process, the interrelationship of conductive particles within each individual aggregate or unit is unaffected, or relatively un-- affected by such subsequent manufacturing steps as are required to make the resistor.

The powder so obtained, to be hereafter referred to as strait jacketed conductive particles may then be added to various mixtures of partially polymerized resins which may be free of conductive particles to form a liquid, semi-liquid or plastic mix which may subsequently be processed into the finished solid resistors.

In summary, therefore, the new and improved process of manufacturing a resistance element in accordance with the present invention comprises the steps of adding conductive particles selected from the group consisting of carbon and metal particles to a solvent solution of a polymerizable resinous material, removing said solvent, polymerizing said resinous material, pulverizing the resulting residue to form discrete units, dispersing the discrete units in a polymerizable resin, forming the mixture into a desired shape, and curing the said polymerizable resin.

In general, any type of solid resin of either inorganic or organic nature, natural or synthetic, which is capable of serving as a dielectric material may be used in the practice of the present invention. Included among the various polymerized solid dielectric materials which may be used are: polymeric paraffins, such as polyethylene; halogenated polyethylenes, such as polyvinyl chloride, polychlorotrifluoroethylene, and polytetrafluoroethylene; polyvinyl esters, such as polyvinyl acetate, copolymers of vinyl chloride and vinyl acetate; polyvinyl esters of polybasic acids, such as obtained from allyl alcohol and maleic anhydride; polyolefin aldehydes, such as polymerized acrolein; polyacrylic acid derivatives, such as polymerized methyl methacrylate; polystyrene; polyvinyl amines, such as polyvinyl carbazole; polymeric acid anhydrides, such as obtained by heating a dibasic acid, such as adipic acid with acetic anhydride or acetyl chloride; polymeric esters, such as obtained by esterifying terephthalic acid and ethylene glycol; alkyd resins, such as obtained from glycerol and a polybasic acid such as phthalic or maleic acid; polymerized fatty oils, such as polymerized linseed oil; epoxy resins, such as obtained by reacting a dihydroxy compound such as bisphenol-A with an epihalohydrin, such as epichlorohydrin; synthetic rubbers, such as polybutadiene, copolymers of butadiene and styrene or acrylonitrile; polymeric Z-chlorobutadiene; polychloroprene; neoprene, I polymeric sulfides, such as vulcanized ethylene polysulfide; polymeric sul fones, such as the addition product of sulfur dioxide and pentyne; polymeric imines, such as polymerized ethyleneimine; polymeric amides, such as those polymers and super polymers obtained from e-amino caproic acid; and those produced from adipic acid-tetramethylene dicar= boxylic acid and hexarnethylenediamine; polyurethanes, such as those obtained by reacting a bivalent isoclyanate, such as tolulene di-osocyanate with a dihydr'oxy alcohol, such as ethylene glycol; formaldehyde-urea reaction products; phenol-formaldehyde condensation products; phenol-furfura-l condensation products; formaldehydeaniline condensation products; polycarbonates, such as the polymeric reaction product of 2,2(4,4 di-hydroxy diphenyl) propane and phosgene; organo polysiloxanes, such as polydimethyl siloxane; coumarone-indene resins, such as obtained from coumarone and indene; ketone resin, such as polychloroacetone; amine-aldehyde resins, such as obtained from the reaction product of benzidine with terephthaldehyde; inorganic resins, such as polyphosphonitrilic chloride; natural resins, such as shellac.

A variety of catalysts may be employed to accelerate or decelerate the rate of polymerization. Included among the many well-known catalyst materials which may be used for the purposes of the present invention are mineral acids, such as hydrochloric, sulfuric or phosphoric; alkaline catalysts, such as sodium hydroxide and amine type boron tri-fluoride complexes; peroxides; ozonides; acid anhydrides; percarbonates, perborates and metallic oxides.

In general, the conductive particles referred to above may consist of either carbon particles or metal particles, or a mixture of both. Accordingly, any metal powder may be used as the conductive material of the present invention such as, for example, copper, iron, zinc, aluminum, magnesium, tin, antimony, silver, chromium, etc.

With respect to carbon, certain forms have been found to offer lower resistance values, such as crystalline carbon graphite), whereas the amorphous forms of carbon (carbon black) afford higher values of resistance. Lamp black, having a specific resistance less than carbon black and greater than graphite may be used to obtain resistances of an intermediate value.

In order that those skilled in the art may better understand how the new and improved resistance material of the present invention may be obtained, the following examples are given by way of illustration and not by way of limitation.

Example 1 Four grams of Durez 7522, a liquid, one step thermosetting phenol-formaldehyde resin, having percent of solids at C. and a viscosity of LOGO-3,000 centipoises at 25 C. were admixed with 800 cc. of butyl Carbitol acetate (diethylene glycol monobutylether) (having a specific gravity of about 1), and 24 cc. of ethyl oxalate were added to this mixture. The resin solution was continuously stirred and small amounts of denatured ethyl alcohol were added until a clear solution was obtained. 400 grams of carbon powder were then added to the solution while the solution was stirred, and stirring was continued until the added powder was thoroughly wetted and dispersed into a smooth homogeneous mixture. The resulting paste was then passed through a Kent mill six times at 30 lbs. force between the rolls. After the last milling step, the resulting paste was put on large plates, placed in an air circulating oven, and subjected to a heat treatment at 150 C. for 32 hours. As a result of this treatment, the volatile ingredients contained in the mixture were removed and the resin was polymerized. The solid product was subsequently pulverized and the resulting powder passed through a 14 mesh screen. Analysis of this powder indicated its composition to be 99.04 percent carbon and 0.96 percent polymerized resin. This powder was set aside, and was labeled Strait-jacket No. 7299 R.

68.4 cc. of butyl Carbitol acetate were added to 295.2 grams of Durez No. 7522, and 16.4 grams of ortho cresol added to the mixture. The resulting solution was ball milled overnight in a one pint ball mill, removed from the mill, set aside and labeled Solution F.

Eighty parts by weight of spindle oil were then admixed with twenty parts by weight of lithium stearate. This mixture was set aside and designated as Lo-Hi.

Thereafter, 7.5 grams of the powder designated 7299 R were added to 95.0 grams of Solution F and 4.0 grams of whiting, an inert filler, added to the mixture. In order to aid in the printing of this material onto a non-conducting base, 3.5 cc. of Lo- -Ii were also added. The ingredients were then admixed by hand and subsequently passed through a Kent mill having a 200 lb. force between its rolls.

Standard resistor strips, having dimensions of 0.186 in length by 0.062 inch in width were thereafter printed using the resulting material (which was stored at 25 to 30 F. to prevent premature polymerization of the Durez resin at room temperature). Each standard resistor had' a resistance value of one megohm. A summary of the complete formulation is given below.

It will be noted that the ratio of total resin to carbon in the cured mixture is 9.65 to 1.

The electrical characteristics of the finished resistor made in accordance with Example 1 are enumerated in 1 Change in resistance as a percent of resistance at +72 F.

2 Change in resistance, as a percent of resistance at 30 volts, per one volt change in applied voltage in the range of voltages between 30 volts and 300 volts, or the highest rated voltage of the resistor.

3 Measured at the rated voltage of the resistor or at 300 volts whichever 'is smaller.

Example 2 By the method of Example 1, a mixture was made using 400 grams of carbon powder, 800 cc. of butyl Carbitol acetate, 24 cc. of ethyl oxalate, 12 grams of Durez 7522 and cc. of denatured ethyl alcohol. The resulting paste was dried at C. for 32 hours in order to remove the volatile constituents and to polymerize the resin. The solid product, having a composition of 97.18 percent carbon and 2.82 percent resin was subsequently pulverized, and the powder resulting from the pulverization was set aside and designated Strait-jacket 7-999 R.

Solution B was prepared by admixing 81.1 grams of Durez 7522 with 13.6 cc. of butyl Carbito-l acetate and 4.16 cc. of ortho cresol. The resultingsolution was ball milled overnight in a one-pint mill.

Thereafter, 12 grams of the powder designated Straitjacket 7999 R were added to 88 grams of Solution B and 3.5 cc. of Lo-Hi (prepared in accordance with Example 1) were added thereto. Additionally, 9 cc. of butyl Carbitol acetate were added to the mixture. The resulting mixture was subsequently passed through a Kent mill, and standard resistor strips, of similar dimensions as those described in Example 1, were printed on a non-conducting base. Each standard resistor had a resistance of 5 megohms.

A summary of the complete formulation is given in Table IV below:

It is noted that the ratio of total resin to carbon in the cured mixture prepared in accordance with Example 2 is 6.01 to 1.

The electrical characteristics of the finished resistors made in accordance with Example 2 are enumerated in Table V below:

Mix #5-9 +6. 0% +1.1% .015% per 5. 3 microvolt. 2 volttzs per 1 Change in resistance as a percent of resistance at +72 1 Change in resistance, as a percent of resistance at 30 volts, per one volt change in applied voltage in the range of voltages between 30 volts and 300 volts, or the highest rated voltage of the resistor.

Measured at the rated voltage of the resistor or at 300 volts, whichever is the smaller.

Table VI "below illustrates the improved electrical characteristics of two additional standard resistors made in accordance with the present invention.

of aggregates of carbon particles more or less coated by polymerized resins, and bonded thereby, in accordance with this invention. In this mix resin is approximately 1% of said aggregate. 155 (6). 4.1/1 Carbon added in the form 1 2.

of aggregates of carbon particles more or less coated by polymerized resins, and bonded thereby, in accordance with this invention. In this mix resin is approximately 4% of said aggregate.

1 This resin was Diirez #7522, same as the main resin component oiinix.

In the manufacture of resistors of the type whichare printed on an insulating base by the screen method, it is important, from the standpoint of obtaining acceptably low deviations from standard resistance values, that the resin to conductor ratio be relatively low, as in mixes 149 (6) and 155 (6) of Table VI above, and in any case lower than twelve. Higher ratios can be employed if inert fillers are used, in order to clean the screen and reduce the tack. 1

Thus, it becomes apparent that the present invention permits, by means of a very limited change of resin to conductor ratio, the manufacture of a family of resistors composed of the same resins and the same conductor particles covering a wide range of resistivities. Attention in this regard is called to a curve shown in FIGURE 1, which presents pertinent data obtained from various standard resistors made in accordance with the present invention. It is noted that the physical dimensions of the resistors employed to obtain the data presented in FIGURE 1, were constant, and the materials used in each case were the same, and further, no fillers were employed.

In FIGURE 1, curve A indicates a relationship between resistance and the resin to carbon ratio of a mix containing Sterling 99 R carbon incorporated therein in the condition as received. Curve B indicates the same relationship when Sterling 99 R carbon is added to the resistor mix in the form of discrete units, or strait jacketed conducting particles of carbon, more or less coated and bonded by a polymerized resin in accordance with the present invention. In the case of curve B, the proportion of the resin binder in the discrete units that were employed was 1 percent. Curves C, D and E indicate compositions that differ from curve B only in that the polymerized resin proportion in the strait jacketed carbon particles was 2, 3 and 4 percent respectively. (The proportion of resin employed in the carbon in the strait jacketed carbon is shown in brackets on each of the curves represented in FIGURE 1.)

It becomes readily apparent from the data presented in FIGURE 1 that the present invention permits the manufacture of a family of resistors from mixes composed of the same materials which have resistivities ranging from 130,000 ohms to 36 megohms with no change in the ratio of total resin to carbon (6 to 1); likewise from 130,000 ohms to 360 megohms within the relatively narrow range of total resin to carbon ratio of substantially 6 to 1 to 9 to 1.

In the experiments performed in order to obtain the an effect on the resistivity of the completed resistor. It

8 data presented in FIGURE 1, all conditions with the exception of the proportion of resin used in the strait jacketed carbon, and changes in the ratio of resin to carbon inthe printing mixture (both as shown), were maintained constant. It should be noted at this point that in this connection, alterations in formulation other than the proportion of resin in the strait jacketed carbon and the resin to carbon ratio in the printing mixture may have is further emphasized that resistance values are, of course, affected by the weight of the printing, and the cycle of curing or polymerizing the resin in the completed resistor. In order to obtain comparable results, therefore, it is suggested that such conditions be maintained constant, as shown above.

As indicated in FIGURE 1, the higher the proportion of resin in the strait jacketed carbon, the higher will be the resistivity of the finished resistor, other things being held constant. In one experiment, in using the same ma terials and methods as indicated above, a 6 percent pro portion of resin was employed in the strait jacketed carbon and a resistor was provided having a resistivity of greater than 20,000 megohms. It was noted that the final resin to conductor ratio of that resistor was found to be 6 to 1, a ratio quite acceptable from the manufacturing point of view.

Thus, a significant advantage of the present invention is that specific conductive materials which, in suitable mixtures, yield desired electrical properties such as those pertaining to resistivity, voltage, temperature coefficients and noise factors, can be incorporated into mixture formulations having proportions of conductive material to non-conductive material which are favorable in securing optimum qualities in the electrical characteristics which are aifected by the choice of proportion, and which are also favorable from the standpoint of subsequent manufacturing techniques. Moreover, by adjustments in the relatively small proportions of polymerized resin in the strait jacketed carbon produced in accordance with the present invention which is incorporated into the resistor formulation, and by a small variation in the proportions of conductive to non-conductive materials in the completed resistor mix, an extremely wide range of resistivities of the finished resistor can be obtained without a sacrifice of desirable qualities in electrical characteristics and further without a sacrifice of desirable conditions for eflicient manufacture.

It should also be noted that ball milling, under controlled conditions, of powders which comprise strait jacketed conductive particles made in accordance with the present invention prior to incorporation with the remainmg ingredients in the preparation of a completed resistor will reduce, by certain proportionate amounts, the resistivity of the completed resistor. This procedure applied to strait jacketed carbon particles results in lower resistances which nevertheless may be of a very high range, i.e., 2,000 megohms and above. Table VII below illustrates the eflect of ball milling.

are printed on an insulating base by the methods heretofore used in the prior art, substantial deviations in resistivity and other electrical properties have been traced to variations in time and conditions of storage, and further to slight variations in normal manufacturing techniques. It has been postulated that these deviations arise from changes in the interrelationship of the conductive particles as affected by said variations. A particular benefit of the present invention is that the interrelationship between the conductive particles (which have been subjected to the strait jacketing technique) and which are thus gathered together to form discrete units, is so fixed, that these particles are more stable under variations in time, conditions of storage, and manufacture of printed resistor elements.

A further advantage of the present invention relates to the problem involving the preparation of a fluid resistor mix used in the manufacture of a specific type of resistor. In the prior art, it is usual to prepare the mix, make samples, test, adjust, and in each instance arrive by successive approximations at the proper formulation. Lack of reproducible results has been a characteristic of the methods and the mixes produced by the methods heretofore employed in the prior art. Furthermore, the

instability of the mixes under normal conditions of storage severely limits the size of the batches which may be manufactured in advance of their use. As a direct result, and when using the conventional methods of the prior art, limited batches must frequently be prepared. By the method of the present invention, however, resistor mixes comprising strait jacketed conductive particles may be formulated which have considerably greater reproducibility with respect to electrical properties than were the mixes heretofore obtained by the methods of the prior art. Moreover, the strait jacketed conductive particles prepared in accordance with the method of the present invention may be kept in cold storage for several months Without any significant changes in the electrical properties thereof. Accordingly, the present invention significantly reduces the manufacturing problems attendant in the pro duction of resistor mix preparation in the manufacture of resistance elements.

A further advantage of the present invention relates to the influence of solvents in accentuating the instability of a resistor mix. In the manufacture of resistors, such as those which are printed on an insulating base by screen methods, the printing mixture requires one or more type of solvent to be used. The greater the resin to conductor ratio, the greater the amount of solvent required. Generally, however, solvents accentuate the instability and sensitivity to the changes in manufacturing and storage conditions. To illustrate this phenomenon, data derived from mixes produced by the prior art are shown in FIGURE 2. The components of these mixes are similar to the components of the mixes used in obtaining the data in FIGURE 1, with the exception of the carbon, which in the present instance was a mixture of Halo black produced by Binney and Smith and Sterling LL produced by Godfrey Cabot, Inc. It becomes apparent from the data presented in FIGURE 2 that conditions pertaining to the milling of the printing mixture affect the resistivity of the resistor, and further that solvents accentuate this effect. From the data shown in FIGURE 2, it is concluded that a minimum use of solvents is desirable. The present invention, however, by permitting the use of a lower resin to conductor ratio, also permits the use of lesser amounts of solvents. Since solvents are also required by other methods of resistor manufacture, the present invention has equal applicability in the manufacture of resistors by methods other than those involving screen printing.

It is further emphasized that the stability of the interrelationship between the conductive particles contained in the strait jacketed powder may further be augmented by a modification in the technique of preconditioning the carbon. For example, the residue from which the volatiles have been removed in the process of providing for a strait jacketed conductive powder, may be subjected to a pelletization process, and the resulting pellets pulverized to the desired degree of fineness. Moreover, it is emphasized that the present invention is not limited in application to the manufacture of printed resistor elements. The powders composed of strait jacketed conductive particles produced by the method of the present invention can likewise be employed in the production of injection and compression molded resistors as well as resistors manufactured by other means.

It is to be understood that the present invention is not to be considered limited to any of the specific embodiments herein illustrated and described, but may be used in other ways without departure from its spirit as defined in the forthcoming claims.

What I claim and desire to secure by Letters Patent of the United States is:

l. A resistance material comprising a plurality of discrete units dispersed in a solid dielectric, said dielectric comprising a polymerized resin, and each discrete unit comprising an aggregate of conductive particles selected from the group consisting of carbon and metal particles, said conductive particles being at least partially precoated and bonded together by a solid polymerized resin to form said discrete unit.

2. A resistance material comprising a plurality of discrete units dispersed in a solid dielectric, said dielectric comprising a polymerized resin comprising the reaction product of a dihydroxy phenol and an epihalohydrin, and each discrete unit comprising an aggregate of conductive particles selected from the group consisting of carbon and metal particles, said conductive particles being at least partially precoated and bonded together by a solid polymerized resin comprising the reaction product of a dihydroxy phenol and an epihalohydrin to form said discrete unit.

3. A resistance material comprising a plurality of discrete units dispersed in a solid dielectric, said dielectric comprising a polymerized resin comprising polystyrene, and each discrete unit comprising an aggregate of conductive particles selected from the group consisting of carbon and metal particles, said conductive particles being at least partially precoated and bonded together by a solid polymerized resin comprising polystyrene to form said discrete unit.

4. A resistance material comprising a plurality of discrete units dispersed in a solid dielectric, said dielectric comprising a polymerized resin comprising polyethylene terephthalate, and each dscrete unit comprising an aggregate of conductive particles selected from the group consisting of carbon and metal particles, said conductive particles being at least partially precoated and bonded together by a solid polymerized resin comprising polyethylene terephthalate to form said discrete unit.

5. A resistance material comprising a plurality of discrete units dispersed in a solid dielectric, said dielectric comprising a polymerized resin comprising polytetrafluoroethylene, and each discrete unit comprising an aggregate of conductive particles selected from the group consisting of carbon and metal particles, said conductive particles being at least partially precoated and bonded together by a solid polymerized resin comprising polytetrafluoroethylene to form said discrete unit.

6. A resistance material comprising a plurality of discrete units dispersed in a solid dielectric, said dielectric comprising a phenol-furfural resin, and each discrete unit comprising an aggregate of conductive particles selected from the group consisting of carbon and metal particles, said conductive particles being at least partially precoated and bonded together by a phenol-furfural resin to form said discrete unit.

7. A resistance material comprising a plurality of discrete units dispersed in a solid dielectric, said dielectric comprising a urea-formaldehyde resin, and each discrete unit comprising an aggregate of conductive particles selected from the group consisting of carbon and metal particles, said conductive particles being at least partially precoated and bonded together by a urea-formaldehyde resin to form said discrete unit. a

8. A resistance material comprising a plurality of discrete units dispersed in a solid dielectric, said dielectric comprising the polymeric reaction product of 2,2(4,4 dihydroxy diphenyl) propane and phosgene, and each discrete unit comprising an aggregate of conductive particles selected from the group consisting of carbon and metal particles, said conductive particles being at least partially precoated and bonded together by a solid polymerized resin comprising the polymeric reaction product of 2,2(4,4 di-hydroxy diphenyl) propane and phosgene carbonate to form said discrete unit. 9. A resistance material comprising a plurality of discrete units dispersed in a solid dielectric, said dielectric comprising a polymerized resin comprising a member from the group consisting of alkyl, aryl, and aralkyl polysiloxanes, and each discrete unit comprising an aggregate of conductive particles selected from the group consisting of carbon and metal particles, said conductive particles being at least partially precoated and bonded together by a member from the group consisting of alkyl, aryl an aralkyl polysiloxanes to form said discrete unit.

10. A resistance material comprising a plurality of A discrete units dispersed in a solid dielectric, said dielectric comprising a polymerized resin comprising polymethyl methacrylate, and each dscrete unit comprising an aggregate of conductive particles selected from the group consisting of carbon and metal particles, said conductive parcomprising a phenol-formaldehyde condensation product to form said discrete unit. 9

13. A resistance material comprising conductive particles of carbon precoated with a liquid, one step thermosetting phenol-formaldehyde resin, having percent of solids at C., and a viscosity of LOGO-3,000 centipoises at 25 C. which has been at least partially polymerized and contained in a polymerized liquid one step thermosetting phenol-formaldehyde resin, having 90 percent of solids at 135 C. and a viscosity of 1,000-3,000 centipoises at 25 C. i

14. The process of manufacturing a resistance element which comprises adding conductive particles selected from the group consisting of carbon and metal particles to a solvent solution of a polymerizable resinous material, removing said solvent, polymerizing said resinous material, pulverizing the resulting residue to form discrete units, dispersing the discrete units in a polymerizable resin free of conductive particles, forming the mixture into a desired shape, and curing the said polymerizable resin.

15. The process of manufacturing a resistance element which comprises adding conductive particles selected from the group consisting of carbon and metal particles to a solvent solution of a polymerizable resinous material, removing said solvent, polymerizing said resinous material, pulverizing the resulting residue to form discrete units, dispersing the discrete units in :a phenol-formaldehyde resin free of conductive particles, forming the mixture into a desired shape, and curing the said phenol-formaldehyde resin.

References Cited in the file of this patent UNITED STATES PATENTS :UNITED STATES PATENT QFFICE CERTIFICATE OF CORRECTION Patent No, 3,056,750 October 2, 1962 Richard H. Pass It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

In the grant, lines 2 and 3, and in the heading to the printed specification, lines 5 and 6, for "Air Reduction Company, Incorporated a corporation of Delaware" read Air Reduction Company, Incorporated, a corporation of New York column 4, line 72, after "monoloutylether" insert acetate Signed and sealed this 5th day of March 1963,

(SEAL) Attest:

ESTON G. JOHNSON DAVID L. LADD kttesting Officer Commissioner of Patents UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No, 3,056,750 October 2, 1962 Richard H, Pass It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below,

In the grant, lines 2 and 3, and in the heading to the printed specification, lines 5 and 6, for "Air Reduction Company, Incorporated, a corporation of Delaware" read Air Reduction Company, Incorporated, a corporation of New York column 4, line 72, after "monobutylether" insert acetate Signed and sealed this 5th day of March 1963,

Claims (1)

1. A RESISTANCE MATERIAL COMPRISING A PLURALITY OF DISCRETE UNITS DISPERSED IN A SOLID DIELECTRIC, SAID DIELECTRIC COMPRISING A POLYMERIZED RESIN, AND EACH DISCRETE UNIT COMPRISING AN AGGREGATE OF CONDUCTIVE PARTICLES SELECTED FROM THE GROUP CONSISTING OF CARBON AND METAL PARTICLES, SAID CONDUCTIVE PARTICLES BEING AT LEAST PARTIALLY PRECOATED AND BONDED TOGETHER BY A SOLID POLYMERIZED RESIN TO FORM SAID DISCRETE UNIT.